Trifluoroethylene (CF2═CHF) is expected as a new refrigerant to replace chlorofluorocarbons (CFC) or hydrochlorofluorocarbons (HCFC) that destroy the ozone layer, or hydrofluorocarbons (HFC) being greenhouse gases.
Such trifluoroethylene is stored or transported as filled in a sealed container under pressure at a temperature of at most normal temperature, or as liquefied and filled in a sealed container under pressure with cooling. Trifluoroethylene filled in a sealed container in such a manner, is in a gas-liquid state having a gas phase and a liquid phase. And, trifluoroethylene in such a gas-liquid state is desired to be kept stably without causing a reaction such as polymerization, in order to maintain the quality as a refrigerant, or to prevent deposition of impurities (solids) in the container.
Fluoroolefins are known to undergo a polymerization reaction if oxygen is present, as the oxygen becomes a radical source. Among such fluoroolefins, tetrafluoroethylene tends to be polymerized in the presence of a very small amount of oxygen at a level of from 1 to a few tens ppm, and in some cases, the polymerization reaction is likely to proceed explosively. For example, Patent Document 1 discloses that tetrafluoroethylene undergoes polymerization at an oxygen concentration of 1.4 ppm to form polytetrafluoroethylene. Therefore, at the time of storing a fluoroolefin, it becomes important to handle it by removing oxygen to the limit.
However, in order to remove oxygen to the limit, it is necessary to take a measure such as providing a new step for removing oxygen to the limit in the production process, whereby the cost associated therewith will be incurred. Further, by conducting the step of removing oxygen to the limit, there may be a case where the yield tends to be low, and the production cost will be increased.
To what extent trifluoroethylene is stable against a self-polymerization reaction in the presence of oxygen, is largely unknown. A method for storing trifluoroethylene without permitting it to undergo a polymerization reaction, is desired, in order to maintain its quality as a refrigerant and to carry out its storage and transportation inexpensively, safely and stably.
Heretofore, for stabilization of hydrofluoroolefins, some proposals have been made. Patent Document 2 has proposed a method of adding a stabilizer such as an alkyl catechol or an alkoxy phenol, in order to maintain the stable state (state without formation of an acid) of hydrofluoropropene even in the presence of air. Further, Patent Document 3 discloses a stabilization method of adding a C1-4 aliphatic alcohol as a stabilizer to hydrofluoropropene.
However, each of such methods disclosed in Patent Document 2 and Patent Document 3 presupposes the existence of a refrigerating machine oil, and it is a method of stabilizing the overall cooling system by stabilizing the refrigerant composition, wherein the conditions are different from stabilization of a refrigerant in a container for storage or transportation, and therefore, it is difficult to apply such a method to store a refrigerant in a container. Further, in the method of adding a stabilizer, it is required to remove the stabilizer prior to use as a refrigerant, whereby not only the load of the process is large, but also there may be a case where the stabilizer cannot be completely removed by a physical purification method such as distillation, such being undesirable from the viewpoint of quality control.